1. Field of the Invention
The invention relates to aircraft gas turbine engine blades retained by dovetail roots in dove tail slots in rotor disks and, particularly, to shims disposed around the dovetail roots.
2. Description of Related Art
Many gas turbine engines retain rotor blades in disks using dovetail roots of the blades disposed in dovetail slots in the rotor. The disks and blades are often made of expensive Titanium alloys because of their good strength, low density, and favorable environmental properties at low and moderate temperatures. Sacrificial shims disposed between the disks and roots are used to reduce fretting and wear of the more expensive disks and roots.
A compressor or fan disk may have an array of dovetail slots in its outer periphery and dovetail roots of a titanium compressor or fan blade is received into each dovetail slot. At rest, the dovetail of the blade is retained within the slot. In one exemplary engine, the shim is retained axially in the disk by a forwardly located blade retainer and an aftwardly located booster spool flange. Past engine experience has shown that during operation, the fan blade shim can move axially in the slot and distress a forward face or surface of the booster spool flange. The booster spool is a critical rotating component and any distress to the surface may render it unserviceable.
When the engine is operating, centrifugal force induces the blade to move radially outwardly. The sides of the blade dovetail slide against the sloping sides of the dovetail slot of the disk, producing relative motion between the blade and the rotor disk. The sliding between the titanium blade root and disk is particularly acute during transient operating conditions such as engine start-up, power-up (takeoff), power-down and shutdown. The sliding can cause fretting of the disk and blade root and lead to a reduction in fatigue capability of the titanium parts. During such operating conditions, normal and sliding forces exerted on the rotor in the vicinity of the dovetail slot can also lead to galling, followed by the initiation and propagation of fatigue cracks in the disk. It is difficult to predict crack initiation or extent of damage as the number of engine cycles increase. Engine operators, such as the airlines, must therefore inspect the insides of the rotor dovetail slots frequently, which is a highly laborious process. Sacrificial shims have been developed to eliminate the wear, fretting, and galling of the titanium disks, rotors, and blade roots.
U.S. Pat. Nos. 5,160,243 and 5,240,375 disclose a variety of single layer and multi-layer shims designed for mounting between the root of a titanium blade and its corresponding groove in a titanium rotor. The simplest of these shims is a U-shaped shim designed to slide over the root of the fan blade, (see FIG. 3 of the '243 patent). The inventors of U.S. Pat. No. 6,431,835 found that a disadvantage to this type of shim are that it has a tendency to come lose during engine operation and also, does not entirely eliminate the fretting between the groove and the fan blade root.
U.S. Pat. No. 6,431,835 discloses a compliant shim for use between the root of a gas turbine fan blade and a dovetail groove in a gas turbine rotor disk to reduce fretting therebetween. The blade root has tabs at its leading and trailing edges that extend radially inwardly from a recessed inner surface of the root. The compliant shim has first and second slots for engaging the tabs. The slots and tabs cooperate to hold the shim during engine operation. An oxidation layer covers the compliant shim.
The blade is mounted to the disk by sliding the shim onto the root and then inserting the shimmed blade into a dovetail slot. The cross-section of such shims do not match the cross-sections of the roots and, thus, sliding the shim onto the root is difficult and can break or weaken the shim. Thus, it is desirable to have a shim that can be easily mounted onto the blade root and requires spreading apart the shim so that it can fit over the dovetail portion of the blade root and snap fit into the slot between the tabs and against the recessed inner surface of the root.